It is difficult to separate the culture from the consequences of the culture. In the US there is minimal social stigma for trying something ambitious and failing, the culture respects the ambition and courage to try even if it doesn't work out or the idea was slightly idiotic. In many parts of the world, trying something and failing is held against you and becomes a social black mark; Americans just shrug and move on to the next thing. Investors in the US reflect that culture.

A lot of American cultural difference relative to the rest of the world can be explained almost entirely by the unusual absence of Tall Poppy Syndrome[0] that is a hallmark of most other industrialized societies. American culture respects those taking large risks for concomitant rewards, and doesn't begrudge the rewards that might accrue to someone taking those risks. It is easy to have a high risk appetite when it increases one's social status.

[0] https://en.wikipedia.org/wiki/Tall_poppy_syndrome

Availability of capital for investors is one thing.

A lot of the VC pipeline also reaps rewards from how USA was the one country with untouched industry at the end of WW2 and then funneled huge amounts of moneys into expanding, not reconstruction.

I have to imagine there are simply more people with so much money that they can throw it around easily. Isn't the whole thing that (for example) when a big startup gets bought you suddenly have loads of millionaires with money to burn in their pocket?